Unlike the fuel cells employing liquid electrolytes such as the phosphoric acid fuel cell (PAFC) and the molten carbonate fuel cell (MCFC), the solid electrolyte fuel cell is free of leakage trouble and does not require refills so that it is expected to be maintenance-free. The solid electrolyte fuel cells heretofore available can be classified, according to the electrolyte characteristics, into the low-temperature type cell which is operated at a temperature not exceeding about 200.degree. C. and the high-temperature type cell which is operated at a temperature of about 1000.degree. C.
Regarding the low-temperature solid electrolyte fuel cell, the cell utilizing a solid polymer electrolyte (SPE), which is a protonically conductive cation exchange membrane, has already been partially implemented for space and other applications. However, since this fuel cell is driven at a low temperature, its energy conversion efficiency is low and the compatible fuel is also limited to hydrogen gas. Moreover, since a large amount of platinum catalyst must be used in the positive and negative electrodes, the cell is too costly for civil applications.
Regarding the high temperature-operated solid electrolyte fuel cell, research has been underway for some time on cells utilizing solid oxides and these cells are generally known as SOFC. As an electrolyte for the SOFC, yttria stabilized zirconia (YSZ) is a favorite subject of research today. The SOFC employing this electrolyte, which exhibits excellent oxide-ionic conductivity at high temperature, is generally operated at 1000.degree. C. The open-circuit voltage of the SOFC at 1000.degree. C. is as low as about 1.0 V but since it is driven at a high temperature, this SOFC provides a high output and can be applied to cogeneration utilizing the quality high-temperature spent heat so that this fuel cell is expected to serve as an energy converter with high energy conversion efficiency. Furthermore, this fuel cell is versatile in the compatible fuel and does not require an expensive catalyst for positive and negative electrodes, thus offering a potential of cost reduction in future exploitation. On the other hand, for the very reason that it is of the high-temperature type, many kinds of ceramics are used for the components of the cell proper and the peripheral parts such as shield members from the standpoint of heat resistance.
For example, generally alumina type reinforced ceramics are used for the cell housing, a perovskite type electronically conductive oxide based on porous LaCoO.sub.3 for the positive air electrode, and a porous cermet of nickel and stabilized zirconia for the negative fuel electrode. However, these component materials have the drawback that they are deficient in mechanical strength and low in long-term operation reliability. Moreover, the cermet air electrode is inferior to the metal electrode in electrical conductivity. With regard to the cermet fuel electrode, it is for suppressing the aging in performance due to oversintering of metallic Ni that the ceramic stabilized zirconia is added but this addition leads to an increased polarization, thus frustrating the effort to improve the discharge characteristic.
To solve those problems, it might be contemplated to lower the operating temperature to a range of 850.degree. to 650.degree. C. and increase the proportion of metallic materials, which are tough and high in electrical conductivity and mechanical strength, for the components and parts of the fuel cell. However, the conventional electrolyte YSZ suffers a marked decrease in ionic conductivity as the temperature is decreased to 850.degree. C. or lower. Therefore, development of a solid electrolyte material showing high ionic conductivity even in a temperature range of 850.degree. to 650.degree. has been earnestly demanded. Furthermore, a molding method by which such electrolyte can be provided in the form of a thin layer has been considered to be an important subject of study.